The present disclosure relates to a semiconductor structure and particularly to field effect transistors having workfunctions modulated by gate strain, and methods of manufacturing the same.
Metal gate field effect transistors require work function modulation in order to tune threshold voltages. Typically, p-type field effect transistors require that the workfunction of the gate electrode be near the mid-band gap or the valence band edge of the semiconductor material of the channel, and n-type field effect transistors require that the workfunction of the gate electrode be near the mid-band gap or the conduction band edge of the semiconductor material. Further, multiple threshold voltages are employed within a set of field effect transistors to provide devices tailored for various purposes. For example, some field effect transistors may be optimized for high performance, and some other field effect transistors may be optimized for low power consumption.
Workfunction modulation by introduction of dipole moments in the dielectric materials, and by altering metallic alloys is known. However, use of these techniques present problems of mobility degradation, processing temperature limitations, and cross-diffusion of gate regions with differing desired workfunction values. These problems become severe in short gate length devices such as devices having gate lengths less than 22 nm.